A 30 year old guy reincarnated to a fantasy world as a demon. Using his knowledge and past experience, he decided to live a fulfilling life but it didn't go the way he planned it and instead he became one of the demon lord's minion. 1435 R.W. (Race War) He was guarding the first floor of the demon lord's castle when the hero's party came. The hero's party lost to him but he didn't kill them and instead trained them so that they can fight the demon lord and end the war.

my english is not that good so bare with me here…